A sloop is the name of a sail-plan of a sailing ship that has been optimized for upwind sailing. The sail plan has a single mast, a forward triangular staysail[?] called a jib, and a rear triangular mainsail[?] controlled by a boom. Sloops are very popular with amateur sailors and yachtsmen, and for racing. The rig is simple, at least in its basic form yet, when tuned properly, maneuverable and fast.

Rationale behind the sloop rig
When attempting to build an efficient ship, an important consideration must be kept in mind. This is the question of whether the ship will need to sail predominantly down-wind (i.e. with the wind behind the boat) or up-wind (with the wind blowing into the boat from the prow). Sloop rigs are specifically designed to optimise upwind sailing.

It is, however, clear that the most difficult direction to sail a (sailing) ship is upwind. Sailing to windward (known as sailing close-hauled) requires some specific design features. Firstly, to make a good upwind sailing ship, the sail must be as vertical as possible to maximise the wind's energy in the sails.

Two forces act on a sailing ship to deviate it from vertical. Firstly, the weight of the rig itself will tend to pull the sailboat over in one direction the moment the ship departs from the vertical. Secondly, the force of the wind acting on the sails makes the mast behave like a lever. The first effect can be counteracted by reducing the weight of the rig itself. Therefore, the sloop rig weighs quite a bit less than other rigs because its simple design requires fewer ropes and sails.

In terms of counteracting the shear introduced by the wind blowing against the sails, a keel is attached to the hull of the boat. A particular sail-plan will have an optimal set of keel shapes and sizes which will reduce the effect of the wind.

When sailing upwind, it is also important to minimize the drag of the wind on the sail and rig. A major cause of drag of the sail is a vortex of turbulent air generated by the top of the mast and sail. Secondary causes are non-optimal aerodynamic shapes of masts, stays and control lines. The sloop minimizes the tip-vortex's drag by making the sail high and narrow, maximizing the amount of sail for the same sized tip-vortex compared to a square-rigged or gaff-rigged ship. Also, the simplicity of the rig reduces the drag induced by control lines, masts and booms.

Sails carried
To maximize the amount of sail carried, the classical sloop may use a bowsprit[?], which is essentially a fixed boom that projects from the front of the boat. For downwind sailing, the staysail may be replaced (or sometimes supplemented) by a spinnaker[?] or genaker[?] of larger sail area. The jib foresail, which does not overlap the mast, may be replaced by a Genoa jib, which overlaps the mast by up to 50%, the mainsail and Genoa thus forming an efficient double wing.

Sloops in their modern form were developed by the French to run British blockades.

They were later adapted to pilot boats (small ships that take a pilot out to a ship to guide it into a harbor). Later still, they were adapted to smaller revenue cutters, exemplified by the Bluenose.

In the 1920s, racing sloops were developed into extremes in the amount of sail they would carry. The "J-boats" became infamous for capsizing, although in good weather they were very fast. These excesses led racing authorities to establish rules for racing yachts, intended to make them fully seaworthy.

The state of the art in racing sloops today may be seen in the 12-meter yachts sailed in the America's Cup competition. The name derives from the formula used to establish if a yacht conforms to the rules for its design-class.